1. Field of the Invention
This invention relates to the field of digital modulators/demodulators (modems) using an electric network as a transmitting medium. The invention more specifically relates to allocating a channel to a transmission between at least two modems connected to the same electric network.
2. Discussion of the Related Art
Modems using an electric network as a transmitting medium are used in particular in home automation applications. In such applications, different electric devices (heating, lighting, alarm, etc.) connected to the network are associated with modems and thus are likely to be controlled by a central control station according to a timed scenario and/or various types of detectors (temperature, fire, inundation, house-breaking, etc.). The central station is generally associated with a system for receiving instructions transmitted by the telephone network so as to control or program scenarios remotely from the home automation installation.
In such an installation, the electric network at 50 or 60 Hz is the transmitting medium. A frequency shift keying (FSK) modulation is generally used. The data to be transmitted is encoded in a binary form, then converted into an analog signal, for example, as samples of portions of one or the other of two frequencies fa and fb which constitute the signal to be transmitted on the network. The amplitude of the modulation is low (around one volt) with respect to the voltage of the power line so as not to harm the supply of the different devices.
Installations using an amplitude shift keying modulation can also be found. Here, the binary data is converted into an analog signal, as samples of portions of a frequency which constitutes the signal to be transmitted on the network.
A disadvantage of existing modems is that they do not allow the presence of several simultaneous communications on the same network between different modems. Indeed, as these communications are all transmitted by FSK modulators provided for operating on two frequencies f.sub.a and f.sub.b, the messages are then mixed and cannot be properly decoed any longer.
Another disadvantage of existing modems is associated with the use of the electric network as a transmitting medium. Indeed, the signal received by a given modem is likely to be strongly attenuated due to the transmitting medium used. This attenuation, which is variable and uncontrollable, essentially has two causes.
First, the network impedance at the frequencies used for the modulation varies according to the loads it supplies. This variation of impedance which is substantially inductive and, for example, varies between 1.5 and 80 .OMEGA. at the frequency of 100 kHz is uncontrollable since it depends on the loads supplied in the installation involved and on the impedance of the electricity dispensing transformer to which the installation is connected. Moreover, the network impedance depends on the loads supplied by all the installations that the transformer supplies. The attenuation due to these variations of impedance is, for example, between approximately 2 and 40 dB for a typically resistive load.
Second, the electric connections attenuate the signal. This attenuation is even more significant if the signal transmitted by a given modem is addressed to a modem that is connected to another branch of the electric circuit. In other words, the crossing of the prepatch panel and of the fuses and circuit-breakers it includes causes a significant attenuation of the signal (of approximately 20 to 30 dB).